1. Field of the Invention
The present invention relates to novel apparatus and processes for the injection of oxygen into a rotary kiln. More particularly, the present invention relates to apparatus and processes which significantly improve combustion in a rotary kiln used for the calcination of minerals such as cement, lime, dolomite, magnesia, titanium dioxide, and other calcined materials
2. Brief Description of the Related Art
In recent years, demand for cement and other calcined materials has outstripped production. In the construction industry, delays to building and transportation improvement projects have been caused by lack of sufficient cement.
The introduction of oxygen into a combustion space, e.g., a furnace, is used in a variety of industries for enhancement of the combustion process. To date, the use of oxygen in rotary kilns has been applied in three main ways, well documented in the literature: introducing oxygen into the primary air, i.e., into the main burner; the utilization of an oxy-fuel bumer in addition to a standard air-fuel burner; and oxygen lancing into the rotary kiln, particularly in a region between the load and the flame, for improved flame characteristics. One of the more documented uses of oxygen in rotary kilns is described in Wrampe, P. and Rolseth, H. C., "The effect of oxygen upon the rotary kiln's production and fuel efficiency: theory and practice", IEEE Trans. Ind. App., 568-573 (November 1976) (incorporated by reference in its entirety herein), which indicates that production increases above 50% produce excessive temperatures into the kiln, but, below this level, kiln operation takes place without major problems.
Each method of introducing oxygen into the cement plant has its advantages, as well as disadvantages. Thus, the introduction of oxygen into the primary air limits the total amount of oxygen capable of being introduced into the kiln, as modern cement kilns utilize 5-10% of the total air used as primary air. Therefore, in order to introduce a meaningful amount of oxygen into the kiln, it is necessary to significantly increase the concentration of oxygen in the air-fuel stream. Increases in the oxygen concentration leads to potential safety problems, since the fuel is in contact with the O.sub.2 enriched air prior to its arrival into the kiln's combustion space, and therefore can burm too early, and or even result in explosions.
The use of a separate oxy-burner represents a more involved solution to increase the thermal transfer to the load, which in general requires significant quantities of quality fuel, such as natural gas or oil, as well as important modifications in the kiln back wall. This method has been previously proposed, such as U.S. Pat. No. 3,397,256 (which is incorporated by reference in its entirety herein). The use of oxygen lances, although a more elegant solution, can locally increase the temperature of the combustion space, which can result in nonuniform heat transfer to the entire flow of clinkers moving through the kiln. Lancing can also produce hot spots in the refractory, which can potentially damage the refractory. The introduction of cold oxygen can lastly limit the beneficial effect of oxygen on combustion, by locally cooling the flame. The employment of lances has been proposed in U.S. Pat. No. 5,572,938, U.S. Pat. No. 5,007,823, U.S. Pat. No. 5,580,237, and U.S. Pat. No. 4,741,694, all of which are incorporated by reference in their entireties herein.
U.S. Pat. No. 4,354,829 describes mixing air and oxygen in a separate pipe, and introducing it through the rotary kiln moving walls. This device suffers from a number of significant problems, which include: the difficulty of creating a leak-free plenum which rotates with the kiln; the difficulty of installing tubes into the kiln; the fact that the air-oxygen mixture is introduced in a location which might actually hurt the combustion process; and the fact that the air introduced in the rotary kiln is cold, therefore introducing additional stresses in the rotary kiln which can damage its very expensive structure from thermal shock.
The general use of oxygen in cement rotary kilns has already been documented to lead to a significant production increase of the kiln, starting with the work of Gaydas, R. A., "Oxygen enrichment of combustion air in rotary kilns," Journal of the PCA R & D Laboratories, 49-66 (September 1965) (incorporated by reference in its entirety herein). Gayclas presents test results from a period between 1960 and 1962. It is mentioned that Geissler suggested in 1903 that oxygen be used for clinker production. Experimental work was done in Germany in the 1940's, but results are not available. If not specifically addressed, a production increase can create various bottleneck regions, such as in clinker cooling equipment or the flue gas exhaust system.
It is one object of the present invention to provide a way of improving the clinker cooler performance in a rotary kiln.
It is another object of the present invention to provide a safe, yet efficient system and method of introducing oxygen into rotary kilns used, for example, in cement producing equipment, in a manner which will enhance flame characteristics and improve production without adversely effecting overall plant operation.